Generally, paper is manufactured by pouring a pulpous slurry over a fine screen so that liquids within the slurry are drained through the screen thus leaving a matt of pulp fibers thereon. The pulp matt is then pressed to squeeze out any remaining liquid and compress the pulp fibers closer together to form a firm sheet. The sheet is then treated to produce a smooth glossy surface.
To obtain the pulpous slurry trees are harvested, debarked and shipped as logs to a grinding facility. The logs are ground between grindstones to dissociate the wood fibers from each other. Through this process however impurities are collected with the pulp, such as teeth from the saws that cut the trees, metal flakes from the debarker, bits of bark, sand, dirt, plastic and other foreign particles from machinery used to process the logs. These impurities must be removed from the pulpous slurry prior to the slurry being passed through the fine screen, otherwise, they would be embedded within the finished paper making it unacceptable for use. As an alternative to grinding, the logs may be chemically broken down into pulp fibers. Also, as an alternative to obtaining pulp from harvested trees, finished paper may be recycled by breaking it down into pulp again. Nevertheless, with these alternative methods, impurities must still be removed from the pulp fibers.
It is well known that impurities are extracted from the pulpous slurry by screens, hydrocyclones or a combination of screens and hydrocyclones. Because these impurities are usually small, their removal by screening alone is not very effective. Therefore, hydrocyclones are often used to extract impurities. Hydrocyclones may be used in series, commonly referred to as a cascade, to increase the percentage of impurities removed while attempting to prevent the waste of acceptable pulp fibers inadvertently expelled with the impurities through their recapture by another hydrocyclone coupled downstream.
Hydrocyclones typically have a conical housing with a reject tip on its lower end, an inlet pipe mounted at an upper end of the conical housing and an accept pipe mounted centrally within the housing. The inlet pipe is mounted generally tangential to the conical housing so that the slurry entering the housing is forced to rotate about the accept pipe towards the reject tip. The rapid rotation of the slurry causes the lighter particles to accumulate at the center with the heavier particles accumulating about the periphery due to the centrifugal forces acting on the particles. The majority of the pulp fibers is expelled from the hydrocyclone through the accept pipe. The majority of the heavy impurities sink to the bottom of the conical housing and are expelled through the reject tip. Because many impurities have similar densities to that of the acceptable pulp fibers, they often must be passed through hydrocyclones several times for their removal.
Another problem which commonly occurs with hydrocyclones is that large, solid objects such as rocks often enter the hydrocyclone which eventually block or clog the reject tip. Also, impurities which are embedded within large clumps of pulp fibers are not effectively extracted from the clumps of pulp.
Accordingly, it is seen that a need remains for a more efficient apparatus for extracting impurities from a pulpous slurry. It is to the provision of such therefore that the present invention is primarily directed.